1. Field of the Invention
The present invention relates to a switching regulator wherein an inputted DC current is turned ON/OFF by a switching element in such a way that an output voltage is controlled. More particularly, it relates to a switching regulator that is configured as a DC/DC converter.
2. Description of the Related Art
Switching regulators controlled by a power source controlling IC are extensively utilized as small power sources, and as DC/DC converters. Such a switching regulator includes a switching element, which turns an inputted DC current ON and OFF, as well as an inductor and a capacitor, which are disposed on the output side of the switching element. A MOSFET, for example, may be employed as the switching element, and controlled by PWM (Pulse Width Modulation).
The switching regulator as described above can alter an output voltage in accordance with an external input signal. In the prior art, in order to vary the output voltage rapidly, the inductance value of the inductor is kept low, and the switching frequency of the switching element is set to be high. Therefore, when a predetermined voltage is to be outputted, the switching frequency sometimes becomes unnecessarily high, which reduces efficiency. When, in order to heighten the efficiency at the output of the predetermined voltage, the inductance value is made high, and the switching frequency is set to be low, changes in output are slowed down.
FIGS. 8A and 8B are diagrams showing relationships between output voltage and efficiency in the prior-art switching regulator as stated above. More specifically, FIG. 8A shows output waveforms including output voltage variations, and the solid line indicates the output voltage in the fast output variation setting, while the broken line indicates the output voltage in the setting where more importance is attached to the efficiency. FIG. 8B shows the efficiencies of each setting for a constant output voltage, where the solid line indicates the efficiency in the fast output variation setting, while the broken line indicates the efficiency in the setting where more importance is attached to the efficiency. As described above, when the output voltage is to be varied rapidly, the efficiency is reduced, and when more importance is to be attached to efficiency, the output change becomes slowed down.
A switching regulator that enhances efficiency at light loads has been proposed (refer to, for example, JP 11-155281 A, which corresponds to U.S. Pat. No. 6,100,675). The switching regulator changes over its switching frequency between light loads and ordinary loads. By way of example, the switching frequency may be set at 100 kHz for light loads and at 500 kHz for ordinary loads, but other values are of course possible without departing from the scope of the invention.
In addition, a switching regulator that can attain high efficiency in a wide frequency region has been proposed (refer to, for example, JP 2004-096982 A, which corresponds to U.S. Pat. No. 7,061,213). The switching regulator performs PFM (Pulse Frequency Modulation) control at light loads, and it changes-over from PFM control to PWM control at heavy loads or upon alteration of the output voltage.
In the prior-art switching regulator described above, however, the inductance value of the inductor is fixed. This poses the problem that, when the inductance value is set to adapt to rapid output changes, the switching frequency becomes unnecessarily high and efficiency is reduced, at the output of the predetermined voltage, and that, when the efficiency at a constant output voltage is set high, the switching frequency is reduced, which slows down changes in the output voltage.
Further, in the system wherein the switching frequency is changed over between light loads and ordinary loads, when a response in the case of changing over the output voltage from a low voltage to a high voltage is to be speeded up for ordinary loads, the switching frequency needs to be further increased. Moreover, the inductor on this occasion needs to transiently feed a charging current for raising the voltage of the output capacitor, in addition to feeding the current to the load. In this regard, especially at a heavy load, the inductor current is used to feed the load even when the ON-time ratio of the switching element is at the maximum, so that charging the output capacitor becomes slow or becomes impossible. Since the inductor is set at a value enhancing the efficiency in ordinary operation, the response cannot be speeded up by causing the current to flow in excess in such a transient response mode.
Further, in the system wherein the type of control is changed over between light and heavy loads or upon the alteration of the output voltage, high efficiency can be attained over a wide frequency region. It is necessary, however, to provide control circuits for the two feedback loops for PFM control and PWM control. Accordingly, a complicated circuit configuration is required, and current dissipation is increased. Moreover, in the same manner as in the preceding system, the inductor is set at a value enhancing the efficiency in ordinary operation, and hence, the response cannot be speeded up by causing a current to flow in excess in a transient response mode.